Gray iron casting process and composition



United States Patent 3,299,482 GRAY IRON CASTING PROCESS AND COMPOSITIONArthur J. Tache, Madison Heights, Mich., and Robert M.

Cage, Indianapolis, Ind., assignors to Chrysler Corporation, HighlandPark, Mich., a corporation of Delaware No Drawing. Filed Mar. 29, 1963,Ser. No. 269,155 5 Claims. (Cl. 22-211) This invention relates toimprovements in the foundry production of gray iron castings forapplications such as engine blocks, brake drums and the like articleswhere good wear properties and high strength are demanded, andespecially to compositions for use in the continuous process of makingsuch castings in a foundry.

In the making of gray iron castings, for example, for engine blocks, thecope and drag halves of the mold flask are rammed with green sand usingany of the many commercially employed methods, for instance, sandslinging, jolt squeeze, diaphragm molding, blow squeeze. The moldcavities are cleaned of loose sand and where applicable a mold wash isapplied and either air dried or dried wtih a gas flame. Individual coresor assemblies of several :cores made by conventional oil sand, shellsand, furan resin sand or other methods are then properly set into themold cavities either manually or through the aid of core settingfixtures. The cope is then placed on the drag half of the mold and heldin place under its own weight or through external clamps, wedges orweights. The mold is now ready for pouring with molten metal.

The metal ingredients of the casting composition may be melted by any ofthe commercially available processes such as cupola, electric furnacessuch as direct arc, indirect are or induction types or an air furnace orcombinations in a duplex type operation.

The typical charge, for example, to a cupola consists of coke, pig iron,steel scrap, foundry returns, cast scrap, briquetted borings, limestone,fluxes of various types and supplemental f-erro alloys to assist inadjusting the silicon,

manganese and in some cases carbon content. These are ture of 2750 F. to2950 F.

The molten metal may be used directly to fill ladles to pour castingsbut to obtain a more uniform composition it is preferred to hold orcollect several tons in a receiver ladle or a holding furnace wherecomposition variations in the cupola iron may be minimized orcompensated for through additions.

The metal is checked for chilling tendency with any one of a number ofaccepted means such as chill wedges or blocks and an inoculation of theiron made during filling of the pouring ladles. The ladles are thentransferred to the pouring line and the molds filled at the prescribedtemperature (2550 F. to 2650 F.) at a proper rate.

After pouring, the molds are slowly transported on the pouring lineconveyor cars to the shakeout station. In the case of cylinder blocksfor example, this takes about hr., suflicient to permit the castings tosolidify. The cope is then removed and the solidified casting is placedon a cooling line conveyor after gate removal. The castings are now at atemperature of about 1600 F. The castings continue to cool for about 1%hours until the core knockout station is reached. The exterior of thecasting is now about 1100 F. or less and the bores still about 1450F.1500 F. The castings are sent over shakeout tables where the burnedout cores and remaining green sand are removed.

, The castings are again hung on the cooling line conveyor and afterseveral hours of cooling bringing the ice I temperature of the castingsdown to about 300 F., are brought by the conveyor to the cleaning andinspection station. Here the castings are cleaned by shot blastingchipping and grinding. They are also usually inspected and gaged andwater tested for leakage in the water jacketed areas.

Satisfactory castings for use, for example, as engine blocks requirethat the hardness of the material be suitable for machining and free ofchilled spots to avoid tool breakage such being possible with a hardnessin the range 170 to 241 Brinell preferably 170 to 229 and that theengine bores also have a hardness above 170 Brinell to avoid a highincidence of bore wear and oil consumption. These results have normallybeen possible with a cupola composition of the following generalcharacter:

Percent Total carbon 3.10 to 3.40 Silicon 1.80 to 2.10 Manganese 0.60 to0.90 Phosphorous Maximum 0.15 Sulfur Maximum 0.12 Chromium Maximum 0.12Iron Balance where the cooling of the castings was properly controlledas described above. However, this has not been always possible andrepeated field complaints of a high incidence of bore wear and oilconsumption on engines revealed on investigation that the bores werequite soft, as low as Brinell and had a microstructure unsuitable forthe type of service to which the castings were to be subjected. Forexample, the microstructure had as much as 50% free ferrite whereas anormal of 5% or less was acceptable. Similar conditions were obtainedwith brake drum castings. There it was essential that a proper wearingsurface of adequate and uniform hardness be obtained without thepresence of hard spots due to chill or primary carbides. Furtherinvestigation showed that most of the soft blocks were either the resultof normal shutdown of the foundry molding and cooling line conveyorsdescribed above such as for lunch hour, shift changes or overnitestoppage or due to unforseeable line stoppages because of equipmentfailures. In each of these instances the castings were retained forextended periods in the mold or on the cooling line conveyors prior tocore knockout. Such permitted the bores to self-anneal because of theslow cooling rate through the secondary graphitizing range ofapproximately 1450 F. to 1200 F.

A solution to a major part of the problem was to continue the linemovement described above over the normal shut-down periods such that themold shakeout and core knockout operations could be accomplished in aminimum length of time. This would require running the foundry withstaggered shifts in the various departments along the foundry molding,shakeout and cleaning lines. This would require additional physicalspace for the accumulation or storage of foundry mold flasks and bankingof blocks after shakeout. It was not practically feasible. Moreover,this approach would not solve the problem of a line shutdown.

The only alternative was to treat or modify the gray iron composition tostabilize the microstructure during the periods of slow cooling tominimize the production of soft bores and poor wearing surfaces on thecastings. In this connection it had been found that these soft castingswere evidenced by the formation of substantial amounts of free ferriteand that control of this action if possible, could be beneficial.

It was known that certain alloying elements such as chromium in therange 0.20 to 0.40% and combinations of chromium, molybdenum, nickel orcopper could alter the microstructure so as to render it more resistantto the effects of heat soaking. Also, it had been suggested as a resultof small laboratory experiments using induction melting with pure ironthat a pearlitic microstructure was possible in gray iron by the use ofa tin alloying element. It was not known, however, what would happen iftin was added, for example, to cupola iron on a continued productionbasis and in high volume or by other commercial procedures or the effectof tin build-up in the base iron. Nor was it known what the effect oftin would be on complex commercial castings from the standpoint of wearand machinability or the ability to reproduce the desired wearproperties in complex commercial castings. Moreover, tin had for manyyears been considered as an undesirable element in iron and steel and tohave an inherent embrittling effect on gray iron and was heldresponsible for any cracks produced in castings.

After considerable experimental work and many, many foundry trials, itwas found that if tin in certain critical quantities in the range 0.04to 0.10 was used in gray iron compositions of the above generalcharacter, that it was possible to obtain satisfactory castingssubstantially free of soft bores and having high wear resistant surfacesby the normal production methods that had previously caused the abovedescribed difliculties when normal or abnormal stoppages occurred in thecasting line. Also, that castings were possible that had a uniformhardness throughout and which were free of chill spots that would makemachining difficult and costly on tools. The amount of tin to use inthis range was dependent upon the carbon equivalent (carbon /3 silicon)the amount increasing with increased carbon equivalency.

Thus, it was found that the problems enumerated above could be entirelyovercome by employing a composition essentially composed of thefollowing ingredients and especially where the castings required highwear resistant surfaces and sections of uniform hardness, the amountsgiven being percent by weight of the total composition:

Percent Total carbon 3.05 to 3.45 Silicon 1.70 to 2.10 Manganese 0.50 to0.90 Phosphorus Maximum 0.15 Sulfur Maximum 0.12 Chromium Maximum 0.15Tin 0.05 to 0.08 Iron Balance By the use of this compositionsatisfactory gray iron castings for engine blocks and brake drums wereobtainable having uniformity of structure and whose microstructure inthose areas of the casting that demand good wear properties and highstrength consists of graphite flakes in a matrix of fine pearlitecontaining not more than 5% uniformly dispersed free ferrite. Thegraphite flakes will be of type A in major amounts with minor amounts ofother types permissible except type C which it is preferred be absent.The graphite flakes will have an average size in the range 4 to 7according to ASTM specification A247. Massive flake graphite whenpresent is conducive to graphite tear-out by boring tools and theproduction of porous appearing articles and is therefore also preferablyavoided. It can be substantially avoided by control of the carbon andsilicon levels as indicated. Moreover, the Brinell hardness range (3000kg.) for this composition will be between about 187-241 BI-IN at ambienttemperature and its minimum ultimate tensile strength will be about32500 p.s.i.

Where the composition is to be used for casting cylinder blocks it ispreferred that the carbon equivalent (carbon /s silicon) be maintainedin the range of 3.76 to 4.15% and that the tin content preferably bebetween .05 to 08%.

The tin addition may be employed in any of its available forms such asmetallic tin but its purity should be such that it is substantially freeof lead, antimony and arsenic i.e., no more than a trace be present asshown by spectographic examination. In order to prevent oxidationlosses, the tin is preferably added to the cupola iron during filling ofthe pouring ladles by the simple addition of cast and preweighed chunksof metallic tin.

When the tin is used in amounts less than 04%, adequate protectionagainst self-anneal during periods of line cessation or breakdown becomequestionable and the ferritic content substantially exceeds the 5%minimum. Hence, at least 04% is essential and .05% preferable forconsistent result as noted above. Between .05 to .08% is found adequatefor normal production operations with compositions within the limits setforth above. In special cases 0.04 to 0.10% may be employed. Tin, inamount above 0.10% may be used but does not add anything to improve themicrostructure of the castings and is found to be economicallyunjustified. Amounts in excess of .25% will result in a loss of tensileand impact strength.

Further, in addition to the benefits described above obtained bymodification of the gray iron composition with tin for foundryoperations, the following were also found:

(a) The repair rate on castings due to internal shrinkage and the needfor impregnation of the castings because of porosity was materiallyreduced.

-(b) Chilled spots in the castings were no longer evident, thus,substantially eliminating tool breakage.

(c) The bore hardness in cylinder block castings was maintainable in amuch narrower and higher range of hardness than that possible where tinwas not used.

((1) The critical tin addition made possible acceptable castings ofchemical analyses which might be expected to be too soft or excessivelyhard.

(e) Oxidation losses of tin in the cupola were found to be extremely lowwith a to recovery of all tin used.

From the above description of the invention it will be evident that theprocess of making gray iron castings substantially free of soft areasand which will provide machinable surfaces of high wearing properties;and substantially uniform hardness is improved by the addition ofcritical amounts of tin to the composition.

It will be understood that all changes and modifications coming withinthe spirit and intent of the invention and the appended claims and allequivalents are contemplated.

We claim:

1. In a continuous process of making gray iron castings not susceptibleto self annealing after pouring in the mold, and prior to core knockout,where said casting are slowly cooled on a moving line subject tostoppages which would cause such self-annealing of the castrugs and asoft matrix in the wear areas 'of the castings having a hardnesssubstantially below Brinell and a microstructure containing in excess of5% uniformly dispersed free ferrite, the step which consists in addingto the molten gray iron composition prior to pouring, between 0.04 to0.25% by weight of the composition of tin to substantially eliminateself annealing and to facilitate the production of castings having amatrix in the wear area thereof of between 170 to 241 Brinell hardnessand a microstructure of fine pearlite containing not more than about 5%uniformly dispersed free ferrite, regardless of said line stoppages.

2. The process as claimed in claim 1 wherein said tin content is between0.04 to 0.10%

3. In a continuous process of making gray iron castings for engineblocks and brake drums not susceptible to self annealing after pouringin the mold, and prior to core knock-out, where said castings are slowlycooled on a moving line subject to stoppages which would cause suchself-annealing of the castings and a soft matrix in the wear areas ofthe castings having a hardness substantially below 170 Brinell and amicrostructure containing substantial amounts of uniformly dispersedfree ferrite in excess of 5%, the step which consists in making saidcastings from a molten metal composition consisting essentially byweight percent of Percent Carbon 3.05 to 3.45 Silicon 1.70 to 2.10Manganese 0.50 to 0.90 Phosphorus Maximum 0.15 Sulfur Maximum 012Chromium Maximum 0.15 Iron -4 Balance and adding to the compositionbetween 0.05 to 0.08% by weight of the composition of tin tosubstantially eliminate self annealing and to facilitate the productionof castings having a matrix in the wear area thereof of between 187 to241 Brinell hardness at ambient temperature and a microstructure of typeA graphite flakes in a matrix of fine pearlite containing not more thanabout 5% uniformly dispersed free ferrite, regardless of said linestoppages.

4. A process as claimed in claim 1, wherein the tin content is between0.05 to 0.08% by weight of the composition.

5. In the continuous process of making gray iron cylinder block castingsnot susceptible to self annealing after pouring in the mold, and priorto core knockout, where said castings are slowly cooled on a moving linesubject to stoppages which would cause such selfannealing of thecastings and a soft matrix in the cylinder bore areas of the castingshaving a hardness substantially below 170 Brinell and a microstructurecontaining in excess of 5% uniformly dispersed free ferrite productiveof soft cylinder bores, the step which consists in preparing a moltengray iron composition having a carbon equivalent of about 3.76 to 4.15%and adding to the molten iron prior to pouring between 0.05 to 0.08percent by weight of the composition of tin to substantially eliminateself annealing and to facilitate the producion of castings havingcylinder bore surfaces of 187 to 241 Brinell hardness at ambienttemperature and a microstructure of type A graphite flakes andsubstantially free of type C graphite flakes in a matrix of finepearlite containing not more than about 5% uniformly dispersed freeferrite, regardless of said line stoppages.

References Cited by the Examiner UNITED STATES PATENTS 1,502,983 4/1921Diefenthaler 130 1,544,562 1/1924 Diefenthaler 75130 3,029,482 4/1962Burnett 22-75 X OTHER REFERENCES Davis et al.: Modern Castings, May1957, vol. 31, pages 96-98.

DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

P. WEINSTEIN. Assistant Examiner.

1. IN A CONTINUOUS PROCESS OF MAKING GRAY IRON CASTINGS NOT SUSCEPTIBLE TO SELF ANNEALING AFTER POURING IN THE MOLD, AND PRIOR TO CORE KNOCKOUT, WHERE SAID CASTING ARE SLOWLY COOLED ON A MOVING LINE SUBJECT TO STOPPAGES WHICH WOULD CAUSE SUCH SELF-ANNEALING OF THE CASTINGS AND A SOFT MATRIX IN THE WEAR AREAS OF THE CASTINGS HAVING A HARDNESS SUBSTANTIALLY BELOW 170 BRINELL AND A MICROSTRUCTURE CONTAINING IN EXCESS OF 5% UNIFORMLY DISPERSED FREE FERRITE, THE STEP WHICH ONSISTS IN ADDING TO THE MOLTEN GRAY IRON COMPOSITION PRIOR TO POURING, BETWEEN 0.04 TO 0.25% BY WEIGHT OF THE COMPOSITION OF TIN TO SUBSTANTIALLY ELIMINATE SELF ANNEALING AND TO FACILITATE THE PRODUCTION OF CASTINGS HAVING A MATRIX IN THE WEAR AREA THEREOF OF BETWEEN 170 TO 241 BRINELL HARDNESS AND A MICROSTRUCTURE OF FINE PEARLITE CONTAINING NOT MORE THAN ABOUT 5% UNIFORMLY DISPERSED FREE FERRITE, REGARDLES OF SAID LINE STOPPAGES. 